feat: add support for local file playback

[superhawk610]

This PR adds support for local file playback. Local files have been visible in the UI, but not playable, since #136.

Resolves #326.

Implementation

The primary struct used to identify a track for playback is MediaPath, which is composed of

• item_id a unique, opaque identifier, meaningful only to Spotify's API
• file_id a unique identifier that points into the local filesystem cache
• file_format an enum indicating what codec this file should be played with
• duration a std::time::Duration indicating the track's length

For remote files hosted on CDN, Spotify's API provides item_id, file_format, and duration, and the app stores file_id if/when it begins streaming the track. For local files, we don't need file_id, since the track already resides on disk, so it's left blank (Default::default()). We're able to guess file_format and duration by using Symphonia's Probe. Local files don't have an item_id on Spotify's end, so we have to generate our own. To do this, this PR includes LocalItemRegistry, which is a singleton that registers PathBufs corresponding to local tracks and associates each with a unique ID that may be used in place of Spotify's item ID.

I can confirm that this works locally, though actually getting Symphonia to playback all tracks is a bit hit or miss (using an album from Bandcamp, about half the tracks work flawlessly and the other half won't play).

[jpochyla]

Thanks a lot, and sorry for the long wait :( I've changed some nits.